In extracorporeal blood treatment, blood is taken out of a patient, treated and then reintroduced into the patient by means of an extracorporeal blood flow circuit. Generally, the blood is circulated through the circuit by one or more pumping devices. The circuit is connected to a blood vessel access of the patient, typically via one or more access devices, such as needles or catheters, which are inserted into a blood vessel access. Such extracorporeal blood treatments include hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, blood fraction separation (e.g. cells) of donor blood, etc.
In extracorporeal blood treatment, it is vital to minimize the risk for malfunctions in the extracorporeal blood flow circuit, since these may lead to a potentially life-threatening condition of the patient. Serious conditions may arise if the extracorporeal blood flow circuit is disrupted, e.g. by an access device for blood extraction (e.g. an arterial needle/catheter) coming loose from the blood vessel access, causing air to be sucked into the circuit which leads to air embolism in the patient and increased risk of clotting in the circuit, or by an access device for blood reintroduction (e.g. a venous needle/catheter) coming loose from the blood vessel access, causing the patient to be drained of blood within minutes. Other malfunctions may be caused by the blood vessel access becoming blocked or obstructed, or by the access device being positioned too close to the walls of the blood vessel access, or by constriction/occlusion of the access device due to clotting.
These malfunctions all originate in a “connection system” between the patient and the extracorporeal blood flow circuit. The connection system includes one or more access devices and possibly one or more releasable connectors for attaching the access devices to tubing in the extracorporeal blood flow circuit.
An apparatus for extracorporeal blood treatment may include one or more surveillance devices that monitor the integrity of the blood flow circuit and issue an alarm and/or cause appropriate action to be taken whenever a potentially dangerous situation is detected. Such surveillance devices may operate on measurement signals from one or more pressure sensors in the circuit. Conventionally, the monitoring during a blood treatment is carried out by comparing one or more measured average pressure levels with one or more threshold values and/or by monitoring the presence of air bubbles using an air detector in the circuit. For example, failure in the blood extraction may involve air being introduced into the circuit, whereby the measured average pressure may approach atmospheric pressure, or the blood flow being blocked or obstructed, whereby the measured average pressure may drop to a low level. A failure in the reintroduction of blood into the blood vessel access due to a failure in the connection system may be detectable as a decrease in the measured average pressure. However, it may be difficult to set appropriate threshold values, since the average pressure in the circuit may vary between treatments, and also during a treatment, e.g. as a result of the patient moving. Further, if an access device comes loose and gets stuck in bed sheets or the patient's clothes, the measured average pressure might not change enough to indicate the potentially dangerous situation.
To increase the monitoring precision, WO 97/10013 proposes detecting, as one of several options, a heart signal in the measured pressure and using the heart signal as an indicator of the integrity of a fluid connection between an extracorporeal blood flow circuit and a blood vessel access. The heart signal represents a pressure wave which is produced by the patient's heart and transmitted from the patient's circulatory system to the extracorporeal blood flow circuit via the blood vessel access. Malfunctions in the fluid connection will disturb the transmission of the heart-generated pressure wave to the circuit, causing the heart signal to change or even disappear. The measured pressure may also include a strong pressure wave produced by the blood pump in the extracorporeal blood flow circuit. In WO 97/10013, the monitoring involves filtering a measured pressure signal to remove the frequency components that originate from the blood pump, and then detecting the heart signal by analysing the filtered pressure signal. The amplitude of the filtered pressure signal is then taken as an indication of the integrity of the fluid connection.
US2005/0010118 proposes a solution which involves applying a frequency analysis to a measured pressure signal to generate a frequency spectrum, and monitoring anomalies of the fluid connection based on the intensity of the frequency component caused by the patient's heartbeat. US2005/0010118 proposes various solutions on how to identify only the frequency component caused by the patient's heartbeat in the frequency spectrum, which consists of a mixture of various frequency components, including those caused by pumps in the extracorporeal blood flow circuit. The proposed solutions all involve a subtraction of a reference frequency spectrum from the frequency spectrum obtained from the pressure signal. The reference frequency spectrum may be obtained from the pressure signal prior to installation of the fluid connection, may be synthesized based on the operating frequency of the pumps, or may be obtained from the pressure signal at a earlier point in time. Irrespective of solution, the result of the subtraction is processed for extraction of a parameter value that represents the intensity of the frequency component caused by the patient's heartbeat. If the parameter value falls below an threshold value, an anomaly of the fluid connection is deemed to have occurred.
All of the above techniques may require significant processing of the pressure signal for removal of pump signals originating from pumping devices in the extracorporeal circuit, and for calculation of a parameter value that properly represents the remaining heart signal. If the filtering fails, so that a portion of the pump signals remains in the signal to be analysed, the parameter value may erroneously indicate presence of a heart signal even if the integrity of the fluid connection is in fact compromised.
The prior art also comprises WO2009/127683, which discloses a technique for monitoring the integrity of an the extracorporeal blood flow circuit in fluid communication with a blood vessel of a patient, by isolating a beating signal in a pressure signal obtained from a pressure sensor in the extracorporeal blood flow circuit. The beating signal manifests itself as a slow amplitude modulation of the pressure signal and is formed by interference between pressure waves generated by the patient's heart and pressure waves generated by a pumping device in the extracorporeal blood flow circuit. Absence of the beating signal is taken as an indication that the integrity of the circuit is compromised. This technique is suited for detection of a heart signal that lies close in frequency to the pump signal, and typically involves filtering of the pressure signal in a narrow passband around a specific frequency component of the pump signal, where the resulting parameter value is calculated to indicate presence or absence of the slowly varying amplitude in the filtered pressure signal.
Corresponding needs to monitor the integrity of a connection system between first and second fluid containing systems may arise in other fields of technology.